With the progress of the genome project, activities to understand the cause of a disease and life phenomenon through understanding an organism at its DNA level have become very active. For understanding the life phenomenon and gene function, studies on gene expression are effective. As a promising method to study this gene expression, probe array, so-called DNA chip, in which a number of DNA probes are immobilized on a solid surface such as slide glass by sorting them out for every kind has come into use. The method of producing DNA chips includes a method in which a nucleotide oligomer with a designed sequence is synthesized base by base in a large number of sectioned cells using a lithography technology widely used in a photochemical reaction and a semiconductor industry (Science 251, 767-773 (1991)) and a method in which a plurality of kinds of DNA probes are spotted one by one to each section (Science 270, 467-470 (1995); Nat. Biotechnol. 18, 438-441 (2000)).
When a DNA chip is produced, the DNA probes need to be immobilized on an array piece by piece or the oligomers need to be synthesized base by base in any of these methods, and its production requires both time and manpower, leading to a high cost. In addition, since the probes are immobilized by applying liquid droplets containing them on a solid surface, there are problems that spot-to-spot variation may result, changing a combination of kinds of the probes is not easy, handling by a user is difficult, and so forth.
In order to solve the above problems, a probe array, that is, bead array in which beads immobilized with DNA probes are prepared and a plurality of kinds of these beads are lined up has been disclosed (Clinical Chemistry 43, 1749-1756 (1997); Nucleic Acids Research 30, e87 (2002); Specification of U.S. Pat. No. 6,023,540). The advantage of the probe array with beads lies in that a probe array without variations in the probe density for each bead can be produced because a method of probe immobilization with the use of a chemical reaction in a solution can be employed.
In the DNA chip, the identification of a probe is performed by way of the location of oligomer synthesis or the spot of each DNA probe. In the probe array having beads immobilized with probes, the identification of a probe is performed by way of beads colored differently for each probe (Clinical Chemistry 43, 1749-1756 (1997); Specification of U.S. Pat. No. 6,023,540) or the order of beads arrayed in a capillary (Nucleic Acids Research 30, e87 (2002)).
For the identification and quantitative analysis of a plurality of kinds of DNA contained in an analyte sample with a DNA chip, the sample is allowed to react with oligomers or DNAs immobilized on the chip over half a day to a day. On the other hand, in a probe array arranged with beads in a capillary, that is, bead array, an analyte sample is forced to flow through the capillary. Since the time required for gene examination with the bead array can be shortened compared with a conventional method, the bead array is a technique of measurement suitable for application at a clinical site such as hospital. For example, the use of bead array can be expected as a means for prompt detection of an infectious disease requiring an urgent diagnosis and a foreign gene non-existent in human and derived from the genome of a pathogenic microorganism in bacteriological examination and the like.
For practical use of a bead array employing a method in which probes are identified from the order of beads arranged in a capillary (Nucleic Acids Research 30, e87 (2002)), it is essential to establish a method for selecting arbitrary beads immobilized with probes according to specific examination purpose and arraying the beads as one desires, and several methods have been proposed for this. For example, there are a method in which beads are poured into a capillary by making use of a liquid flow while controlling individual beads one by one (JP-A No. 243997/1999) and another method in which only one bead is retained on a sheet provided with a fine recess in which only one bead from among a plurality of the beads introduced with a solvent can be put and the sheet is moved to a position of a capillary or a slot provided on a flat plate while retaining the bead, followed by arraying the bead one after another (JP-A No. 346842/2000). However, these methods often fail to incorporate beads because of the influence of air bubbles, and therefore there was a problem in reliability and usability.
Hence, a method in which only one bead is captured at the suction tip of a bead capturing nozzle from among a plurality of the beads immobilized with the same probe with the use of the bead capturing nozzle (JP No. 3593525; JP-A No. 17224/2005; Analytical Chemistry 75, 3250-3255(2003)). According to this method, beads can be arrayed in the order as intended. In order to capture only one bead at the suction tip of the bead capturing nozzle, beads additionally attached to the side surface of the bead capturing nozzle due to static electricity need to be removed. For this purpose, the surface tension of the air-liquid interface that arises at the border of the air and a solution is utilized as a means. The extra beads attached to the sidewall of the bead capturing nozzle cannot be passed through the interface and are retained on the solution side of the interface when the bead capturing nozzle is withdrawn from the solution into the air. Since the beads retained on the interface cannot be dislocated into the air, the beads attached to the sidewall of the bead capturing nozzle slide down along the sidewall and left behind in the solution. As the result, only one bead held via suction is captured by the bead capturing nozzle after withdrawing the bead capturing nozzle into the air. It should be noted that extra beads attached to the tip surface are taken out into the air since the tip surface of the nozzle is not influenced by the force from the air-liquid interface. To prevent this from occurring, it is necessary not to provide any space at the tip surface that allows bead attachment besides the suction portion, and therefore it has been necessary to use the bead capturing nozzle having an outer diameter approximately equal to that of the bead.